This invention relates to a semiconductor optical device apparatus such as a semiconductor laser or a semiconductor optical amplifier.
A structure so-called as a ridge waveguide type is frequently used to easily produce semiconductor optical device apparatuses. FIG. 4 shows a manufacturing method for such a structure.
First, an n-type clad layer 402, an active layer 403, a p-type clad layer 404, and a p-type contact layer 405 are formed on a substrate 401. Subsequently, a photoresist 408 having stripe openings as a pattern made by photolithography is formed on a wafer surface to form a stripe-shaped ridge by a wet etching process using the photoresist as a mask so that the p-clad layer remains with a prescribed thickness. A protection film 409 having insulating property is formed on the whole wafer surface; the protection film at a top of the ridge is removed by photolithography; and a p-side electrode 410 and an n-side electrode 411 are formed. The ridge structure thus formed can make the transverse mode for laser oscillation stabilized and can reduce the threshold currents.
However, with such a conventional manufacturing method for ridge waveguide type semiconductor optical device apparatus, because the ridge portion is formed by an etching, it is difficult to control the thickness of the clad layer in a non-ridge portion 406 with high accuracy. As a result, slight differences in the thickness of the clad layer in the non-ridge portion make the effective refractive index greatly deviated at that portion, thereby making the laser property of the semiconductor optical device apparatus deviated and improvements in product yields not easily obtainable.
To solve such a problem, a method has been proposed in which the thickness of the clad layer of the non-ridge portion is determined using a crystal growth rate during the crystal growth, in which a protection film is formed at the non-ridge portion, and in which the ridge portion is re-grown (see generally, JP-A-5-121,822, JP-A-9-199,791, JP-A-10-326,934, JP-A-326,935, JP-A-10-326,936, JP-A-326,937, JP-A-326,938, JP-A-10-326,945). FIG. 5 shows producing method and structure for such a laser device. When the ridge portion is formed, a layer is selectively re-grown in using a protection film 506 as a mask on stripe shaped openings 507, and a p-type second clad layer 508 and a p-type contact layer 509 are sequentially accumulated with trapezoid cross-sectional shapes according to isotropic nature in the growth rate with respect to face orientation. With this method, the thickness of the p-type first clad layer 504 in the non-ridge portion can be controlled with high accuracy, so that the effective refractive index can be controlled easily.
However, the semiconductor optical device thus manufactured by this method also raises a problem. For example, the ridge waveguide type laser as set forth in JP-A-5-121,822 should have a ridge width around one micron at the ridge top if an optical waveguide structure is manufactured to achieve a single fundamental transverse mode. Consequently, because the contact area between the contact layer and the electrode becomes so small, the contact resistance between the contact layer and the electrode may increase, and laser characteristics and reliability may be deteriorated due to oxidized surfaces of the clad layer at the ridge side wall. Therefore, it is difficult to improve the product yield.
In the case of the ridge waveguide type laser as set forth in JP-A-199,791, because the bottommost portion of the ridge becomes in a reversed-mesa shape, the contact layer may not be formed, thereby raising problems such that the device is easily oxidized and that the life time may be adversely affected. Since the electrode is not easily formed at the bottommost portion of the ridge, the interconnection may be cut, thereby creating a problem that the production yield is adversely affected. Therefore, it is demanded to provide a semiconductor optical device apparatus with high reliability and good yield in manufacturing.
Meanwhile, optical discs are made with a higher recording density these days, and according to this, light sources are developed vigorously. To make smaller the condensed spot diameter on a disc plate, practical use of red lasers (635 to 690 nm), instead of near infrared lasers (around 780 nm), begins, and blue semiconductor lasers having wavelength of around 400 to 420 nm, though in a stage of developments, are about to achieve longer lifetime in a CW operation. On the other hand, to focus the spot on the disc plate by condensing the laser beam, the laser beam is preferably formed in a shape closer to a circular shape, but actually, the beam divergence angle in a horizontal direction in a face parallel to the active layer is about one third in comparison with that in the vertical direction. Generally, a widened light intensity profile at the end face of the laser beam emission in the transverse direction causes the divergence angel in the horizontal direction to be small. A beam having an divergence in a shape closer to a circular shape can be obtained by narrowing the width of the stripe-shaped openings and by making the optical intensity profile at the emission end surface small, but the narrowed width of the stripe shaped openings increases current injections density to the active region, thereby promoting bulk deterioration, and raising a problem that the reliability of the device may be lowered. Particularly, in a material for short wavelength light source such as AlGaInP based, AlGaInN based, and MgZnSSe based materials, this problem becomes serious due to larger bulk deterioration caused by current injections in comparison with the conventional AlGaAs based material. If a beam closer to a circular shape is used, there are advantages such that the laser beam can be used with an improved efficiency (i.e., light amount cut by lenses becomes small) and any correction plate for beam shape becomes unnecessary. Therefore, it is demanded to provide a semiconductor optical device apparatus with a smaller beam spot diameter operable in keeping high reliability.
Since media price can be lowered relatively these days, CD-R (recordable), CD-R/W (re-writable), mini-disc (MD), and the like begin to be commercially available, and therefore, the light source is required to have a largely improved light output (70 to 100 mW in CW) in order to correspond to a high speed operation where made of the conventional AlGaAs (wavelength is around 780 nm). With a conventional art, it is hard to adequately suppress the deterioration in laser, particularly, end surface deterioration, during the above high output operation. It is demanded to provide a semiconductor optical device apparatus with high output and high reliability.
Meanwhile, with respect to the compound semiconductor layer containing In in the semiconductor optical device apparatus, the followings have been known. Because lattice matching should be made to the substrate, the In content of the respective layers of the double hetero structure including an n-type clad layer, an active layer, and a p-type clad layer, like InGaAsP/In(AlGa)AsP/InP based and InGaAs/In(AlGa)As/InP based, which are formed on an InP substrate, and InGaP/IN(AlGa)P based and InGaAs/InGaAsP/InGaP based, which are formed on a GaAs substrate, is designed to have 50% or more. In general, the In content is determined to be a necessary composition to match the lattice for the substrate whereas the Al and Ga content is determined to be a necessary composition to adjust the refractive index and the size of the bandgap. For example, for an (AlGa)InP based red visible light laser (600 nm band) produced on a GaAs substrate, the In content is set about 50% of the entire III group as to make the lattice matching of the active layer and the clad layer substantially with the substrate, and the refractive index and the bandgap are adjusted by setting the Al content in the active layer to be small (generally, Al content is 0%) whereas the Al content in the clad layer to be large (generally, Al content is 30 to 50%). To improve the laser property recently, a quantum well active layer is frequently strained, and in such a situation, the In content is generally varied between 40% and 60%.
To improve recording density of media such as a digital video disc as a center, a visible laser (generally, 630 to 690 nm) using an AlGaInP based material starts used practically as a light source for information processing instead of the conventional AlGaAs (wavelength is around 780 nm), but the following researches have been made to realize shorter wavelength, lower threshold, and high temperature operation.
In a production of an AlGaInP/GaInP based visible laser device, use of a substrate having an off-angle from the (100) plane toward the [011] direction (or [0-1-1] direction) allows to prevent the band gap from narrowing due to formation (ordering) of natural super lattices, thereby rendering the wavelength shorter readily, facilitating high concentration doping of p-type dopants (e.g., Zn, Be, and Mg), and improving the oscillation threshold current of the device by enhancement of the hetero-barrier and temperature characteristics. If the off-angle is too small, step bunching appears outstandingly, and large undulations are formed at the hetero-boundaries, so that a shift amount in which the PL wavelength (or oscillation wavelength) is shortened by quantum effects to the bulk active layer may be smaller than the designed amount where a quantum well structure (GaInP well layer of about 10 nm or less) is manufactured. If the off-angle is made lager, the step bunching is reduced, and the hetero-boundaries become flat, thereby making the wavelength shorter by the quantum effect as designed. Thus, a substrate having an off-angle of 8 to 16 degrees from the (100) plane toward the [011] direction (or [0-1-1] direction) is generally used to suppress formation of natural super lattices and generation of step bunching, which impede the wavelength from becoming shorter, as well as to suppress the oscillation threshold current from increasing due to shortened wavelength from p-type high concentration doping and impairment of temperature characteristics. A proper off-angel should be selected in consideration of thickness and the stress amount of the GaInP well layer depending on the targeted wavelength such as 650 nm or 635 nm. In a meantime, if a substrate having a large off-angle is used for shortening the wavelength, there raises a problem that the horizontally asymmetry of the ridge shape in the ridge waveguide type laser may affect the horizontal asymmetry of the light intensity profile.
Research and development and practical use are made in recent time for optical fiber amplifier, and a large scaled wavelength multiplex transmission system begins to be established in the optical telecommunication field. On the other hand, semiconductor optical amplifiers advantageously can be integrated monolithically with other semiconductor optical devices such as optical switches, modulators, lasers, and the like and have a broader wavelength range, and therefore, research and development orienting applications in the optical telecommunication field is actively made. New applications such as wavelength conversions and optical gates positively utilizing the large non-linearity possessed by semiconductor optical amplifiers start to be researched. However, the optical semiconductor amplifier has a room to be improved in terms of coupling with optical fibers, crosstalk, polarization dependency, noise, and the like, and those create obstacles in actual use.
Various technologies have been developed so far as described above, but the ridge waveguide type semiconductor optical device apparatus still has a room to be improved, and wait for developments of improved technology. It is a basic object to provide a better semiconductor optical device apparatus capable of solving the problems on the prior art as described above. More specifically, it is a first object of the invention to provide a semiconductor optical device apparatus for obtaining a high output during low operation current, with high reliability and high production yield. It is a second object of the invention to provide a semiconductor optical device apparatus having a smaller beam spot diameter, with high reliability and high production yield. It is a third object of the invention to provide a semiconductor optical device apparatus, particularly semiconductor optical amplifier for improving coupling with optical fibers, crosstalk, polarization dependency, noise, a nd the like.
This invention is to provide a semiconductor optical device apparatus at least having, on a substrate, a compound semiconductor layer containing an active layer, a protection film having a stripe-shaped opening formed on the compound semiconductor layer, and a ridge type compound semiconductor layer formed as to cover the stripe-shaped opening having a smaller refractive index than the refractive index of the active layer. The semiconductor optical device apparatus satisfies the following conditions (a) and/or (b);
condition (a): a width (WC) at an opening center of the stripe-shaped opening is different from a width (WF) of the opening front end; and
condition (b): a width (WC) at an opening center of the stripe-shaped opening is different from a width (WR) of the opening rear end.
As a favorable embodiment to accomplish the first object of the invention (hereinafter referred to as xe2x80x9cfirst embodimentxe2x80x9d), a semiconductor optical device apparatus satisfying either or both of WFxe2x89xa7WC and WRxe2x89xa7WC can be exemplified. As a favorable embodiment to accomplish the second object of the invention (hereinafter referred to as xe2x80x9csecond embodimentxe2x80x9d), a semiconductor optical device apparatus satisfying either or both of WFxe2x89xa6WC and WRxe2x89xa6WC can be exemplified. As a favorable embodiment to accomplish the third object of the invention (hereinafter referred to as xe2x80x9cthird embodimentxe2x80x9d), a semiconductor optical device apparatus satisfying WFxe2x89xa6WCxe2x89xa6WR or WFxe2x89xa7WCxe2x89xa7WR, particularly, WFxe2x89xa7WCxe2x89xa7WR can be exemplified.
The semiconductor optical device apparatus according to the invention is useful as, e.g., a semiconductor light-emitting device, a semiconductor laser, and a semiconductor optical amplifier.